The presence of an undesired particle on a photomask may create a corresponding defect in a semiconductor wafer when the contaminated photomask is used for semiconductor lithography. Due to the high degree of miniaturization used on many semiconductor devices, even the smallest of particles may render the resultant semiconductor device useless.
Photomasks are often used multiple times over an extended period of time. In order to keep the photomasks clean between uses, protective pellicle layers are typically mounted above the photomasks. Protective pellicle layers are typically transparent membranes supported by a frame. The pellicle layer is supported a predetermined distance from the photomask so that particulates that fall onto the mask are caught on the pellicle layer and are held out of focus during lithography, so as not to be disruptive.
If a particulate finds its way past the protective pellicle layer, the pellicle layer is typically removed and scraped. The photomask is then cleaned and a new pellicle layer is placed over the photomask. The expense of scraping the original pellicle layer, cleaning the photomask and replacing the pellicle layer adds greatly to the cost of using and maintaining photomasks.
In the prior art record, there are many devices that are used to wash or otherwise clean photomasks after the pellicle layer is removed or prior to the initial application of the pellicle layer. Such prior art devices are exemplified by U.S. Pat. No. 4,715,392 to Abe et al., entitled AUTOMATIC PHOTOMASK OR RETICLE WASHING AND CLEANING SYSTEM and U.S. Pat. No. 4,811,443 to Nishizawa, entitled APPARATUS FOR WASHING OPPOSITE SURFACES ON A SUBSTRATE. The problem with such prior art washing devices is that they only produce a limited percentage of photomasks with zero defects. A higher percentage of photomasks are produced with limited defects. The photomasks with limited defects must either be used in their defective state, scrapped or washed again. The problems associated with washing photomasks multiple times include the time and labor involved in repeatedly performing the washing procedures. Furthermore, defect particles that were not removed by a first washing tend to have characteristics that make those particles impervious to subsequent washings. Recognizing that it is neither cost effective nor time effective to wash photomasks multiple times, supplemental prior art devices have been developed to specifically target and remove defect particles that survive an initial washing procedure. A common prior art approach is to use a laser to burn away any defect particle. Such prior art devices are exemplified by U.S. Pat. No. 4,980,536 to Asch et al, entitled REMOVAL OF PARTICLES FROM SOLID-STATE SURFACES BY LASER BOMBARDMENT and U.S. Pat. No. 5,023,424 to Vaught, entitled SHOCK WAVE PARTICLE REMOVAL METHOD AND APPARATUS. The use of lasers, however, has drawbacks because the removed particle may leave residue when burned or may not burn. Furthermore, on certain photomasks the energy from the laser may be too intense and may harm the photomask before the targeted particle has been removed. The use of lasers, however, has been preferred and its drawbacks ignored, because it is one of the most practical methods of removing a defect from a photomask without having to scrap an existing pellicle layer. The laser is simply focused through the pellicle layer and onto the below lying photomask.
Another prior art method of removing contaminants from photomasks is by the use of pneumatic vibrations. Such a prior art system is exemplified by U.S. Pat. No. 4,677,704 to Huggins, entitled CLEANING SYSTEM FOR STATIC CHARGED SEMICONDUCTOR WAFER SURFACE. Pneumatic systems are complex and often are incapable of removing a firmly placed contaminant particle without damaging the substrate on which the contaminant rests. This method, however, also requires that the pellicle layer protecting the photomask be removed.
Electrostatic forces are also used to remove contaminant particles from photomasks. Prior art electrostatic devices typically apply a charged surface across the entire area of the photomask, hoping to attract any and all contaminant particles present on the photomask. Such prior art systems are exemplified by U.S. Pat. No. 4,744,833 to Cooper et al, entitled ELECTROSTATIC REMOVAL OF CONTAMINANTS. A problem with such systems is that by placing a charged plate above the photomask, particles in depressions on the photomask are exposed to a lesser attraction force than are those particles on the upper most levels of photomasks. Furthermore, the charged plate can only by brought within a given distance of the photomasks to avoid physical contact with the photomask and avoid certain electrical arcing problems. To use electrostatic systems, the pellicle layer must again be removed so that contaminant particles can reach the electrostatically charged surface.
In view of the problems inherent in prior art devices, it is therefore an objective of the present invention to provide a device and method for physically removing contaminant particles from a photomask one particle at a time, thereby positively eliminating all defects without concern of incidental damage to the photomask.
It is a further object of the present invention to provide a device for cleaning a photomask without having to remove and scrap the protective pellicle layer used to protect the photomask.